linux-kernel - Re: What if a TLB flush needed to sleep?

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Message-ID: <20080327080926.GE12346@kernel.dk>
Date:	Thu, 27 Mar 2008 09:09:26 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	Matthew Wilcox <matthew@....cx>, linux-arch@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: What if a TLB flush needed to sleep?

On Wed, Mar 26 2008, Luck, Tony wrote:
> > Of course, someone who wrote it could do better ;-)
> 
> Here is Willy's code in patch format (against linux-next
> tree tag next-20080326 which includes his re-write of the
> semaphore code).
> 
> This looks a lot cleaner than my ia64 specific code that
> used cmpxchg() for the down() operation and fetchadd for
> the up() ... using a brand new semaphore_spin data type.
> 
> It appears to work ... I tried to do some timing comparisons
> of this generic version against my arch specific one, but the
> hackbench test case has a run to run variation of a factor of
> three (from 1min9sec to 3min44sec) so it is hopeless to try
> and see some small percentage difference.
> 
> commit 0359fbb64297d44328f26ec5fda3a3c26f1c5ba7
> Author: Matthew Wilcox <matthew@....cx>
> Date:   Wed Mar 26 11:08:18 2008 -0700
> 
>     Add "down_spin()" API for semaphores
>     
>     For those places that need semaphore semantics but cannot sleep
>     
>     Signed-off-by: Tony Luck <tony.luck@...el.com>
> 
> diff --git a/include/linux/semaphore.h b/include/linux/semaphore.h
> index a7125da..3404ce5 100644
> --- a/include/linux/semaphore.h
> +++ b/include/linux/semaphore.h
> @@ -53,6 +53,12 @@ static inline void sema_init(struct semaphore *sem, int val)
>  extern void down(struct semaphore *sem);
>  
>  /*
> + * Attempt to acquire the semaphore.  If another task is already holding the
> + * semaphore, spin until the semaphore is released.
> + */
> +extern void down_spin(struct semaphore *sem);
> +
> +/*
>   * As down(), except the sleep may be interrupted by a signal.  If it is,
>   * this function will return -EINTR.
>   */
> diff --git a/kernel/semaphore.c b/kernel/semaphore.c
> index bef977b..d3eb559 100644
> --- a/kernel/semaphore.c
> +++ b/kernel/semaphore.c
> @@ -23,6 +23,7 @@
>   */
>  
>  static noinline void __down(struct semaphore *sem);
> +static noinline void __down_spin(struct semaphore *sem);
>  static noinline int __down_interruptible(struct semaphore *sem);
>  static noinline int __down_killable(struct semaphore *sem);
>  static noinline int __down_timeout(struct semaphore *sem, long jiffies);
> @@ -41,6 +42,21 @@ void down(struct semaphore *sem)
>  }
>  EXPORT_SYMBOL(down);
>  
> +void down_spin(struct semaphore *sem)
> +{
> +	unsigned long flags;
> +	int count;
> +
> +	spin_lock_irqsave(&sem->lock, flags);
> +	count = sem->count - 1;
> +	if (likely(count >= 0))
> +		sem->count = count;
> +	else
> +		__down_spin(sem);
> +	spin_unlock_irqrestore(&sem->lock, flags);
> +}
> +EXPORT_SYMBOL(down_spin);
> +
>  int down_interruptible(struct semaphore *sem)
>  {
>  	unsigned long flags;
> @@ -197,6 +213,20 @@ static noinline int __sched __down_timeout(struct semaphore *sem, long jiffies)
>  	return __down_common(sem, TASK_UNINTERRUPTIBLE, jiffies);
>  }
>  
> +static noinline void __sched __down_spin(struct semaphore *sem)
> +{
> +	struct semaphore_waiter waiter;
> +
> +	list_add_tail(&waiter.list, &sem->wait_list);
> +	waiter.task = current;
> +	waiter.up = 0;
> +
> +	spin_unlock_irq(&sem->lock);
> +	while (!waiter.up)
> +		cpu_relax();
> +	spin_lock_irq(&sem->lock);
> +}

This doesn't look very nice - if down_spin() is called with interrupts
disabled, __down_spin() enables them.

-- 
Jens Axboe

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